COMPOSITE MATERIALS

Abstract

A permeable composite material for making an electrode for an electrochemical cell, the composite material comprising: a support defining pores; and alkali metal deposited on the support within a plurality of said pores. An electrode comprising the composite material is also described, as are methods of making the material and cells and assemblies comprising the electrode.

Claims

1-28. (canceled)

29. A permeable composite material for making an electrode for an electrochemical cell, the composite material comprising: a support comprising a fibrous woven or non-woven material defining pores; and an adherent alkali metal vapour deposit on the support within a plurality of said pores, wherein internals of the support with alkali metal vapour deposit thereon define through pores from a first side of the material to a second side of the material.

30. The material of claim 29, wherein the material has a porosity in the range of from 10% to 95% v/v.

31. The material of claim 29, wherein the material has a JIS Gurley value in the range of from 10 to 5000 sec.

32. The material of claim 29, being generally planar with opposed faces defining a thickness of the material, the thickness being in the range of from 15 to 80 μm.

33. The material of claim 29, wherein the support comprises a polymeric material formed from one or more ethylenically unsaturated monomers.

34. The material of claim 29, wherein the support comprises a polypropylene non-woven fabric.

35. The material of claim 29, wherein the alkali metal consists of lithium metal or a lithium alloy.

36. A method of making a permeable composite material for making an electrode for an electrochemical cell, the method comprising: providing a support comprising a fibrous woven or non-woven material defining through pores from a first side of the support to a second side of the support; and vapour depositing alkali metal onto the support within a plurality of said pores to form said composite material, the composite material retaining through pores defined by one or more internal walls of the support with alkali metal deposit thereon.

37. The method of claim 36 comprising physical vapour deposition of the alkali metal.

38. The method of claim 36 comprising evaporating alkali metal and depositing the evaporated lithium onto the support under a vacuum.

39. The method of claim 38 wherein the alkali metal is deposited under a vacuum having a pressure in the range of from 10.sup.−3 mbar a to 0.1 mbar a.

40. The method of claim 39 wherein the vacuum has a pressure in the range of from 10.sup.−3 mbar a to 10.sup.−2 mbar a.

41. The method of claim 36 wherein the alkali metal is deposited at a rate in the range of from 2 to 8 mAh per cm.sup.2 of support per minute.

42. The method of claim 36 comprising evaporating alkali metal under a pulsed application of heat and depositing the evaporated lithium onto the support under a vacuum.

43. The method of claim 36 comprising calendaring the composite material.

44. An electrode for an electrochemical cell comprising a composite material according to claim 29, a connection terminal and optionally a current collector.

45. An electrode assembly comprising an anode, a cathode and a separator positioned therebetween, wherein the anode is an electrode according to claim 44.

46. An electrochemical cell comprising an electrode according to claim 44.

47. The cell of claim 46, wherein the cell is a primary cell or a secondary cell.

48. The cell of claim 46, wherein the cell is a lithium-ion cell.

49. The cell of claim 46, wherein the cell is a lithium-sulphur cell comprising an electrode according to claim 44 as the anode, a sulphur-containing cathode and an electrolyte.

Description

[0106] The present invention will now be further described with reference to the following non-limiting examples and the accompanying illustrative drawings, of which:

[0107] FIG. 1(a) shows impedance data of an electrode according to Example 4;

[0108] FIG. 1(b) shows impedance data of an electrode according to Example 1;

[0109] FIG. 2 is a comparison of overvoltage vs. number of cycles for the smooth lithium electrode of Example 4 and the composite porous lithium electrode of Example 1;

[0110] FIG. 3(a) is a scanning electron microscope (SEM) image of the morphology of the lithium surface of an electrode according to Example 2;

[0111] FIG. 3(b) is an SEM image of the morphology of the lithium surface of an electrode according to Example 3;

[0112] FIG. 4 is a comparison of the impedance data of a lithium half-cell assembled from the 70% porosity electrode of Example 2 and a lithium half-cell assembled from the 45% porosity electrode of Example 3; and

[0113] FIG. 5 is a comparison of the average overvoltage of cycling for a lithium half-cell assembled from to the 70% porosity electrode of Example 2 and a lithium half-cell assembled from the 45% porosity electrode of Example 3.

EXAMPLES

[0114] Experimental Method 1—Porosity

[0115] Unless specified otherwise, porosity values of porous materials mentioned herein are determined according to the following method:

[0116] In a glovebox, under an inert argon atmosphere, a 1 cm.sup.2 square sample was cut out and the thickness of the sample was measured using a micrometer.

[0117] The pores of the sample were washed with polypropylene carbonate (PC). Thereafter the pores of the sample were filled with PC. Further PC washes and vacuum were applied to the sample sequentially, several times, until the formation of micro-bubbles was no longer observed. A spatula was used to remove excess PC from the surface of the sample, and the resulting ‘wet’ sample was weighed.

[0118] The sample was then placed into a vacuum chamber and the PC was removed from the pores under vacuum (between 1.3 to 2.6 mbar a) at a temperature of 40 to 50° C. The sample was dried under vacuum for at least 24 hours, until its weight stabilised. The weight of the “dry” sample was then recorded.

[0119] Using the difference in weight of the sample when ‘wet’ and ‘dry’, the total weight of PC that was present in the pores of the ‘wet’ sample was calculated. The volume of PC present in pores of the sample was then calculated from the weight and known density of the PC.

[0120] The total volume of PC that was present in the pores is equal to the total volume of the pores. Therefore the porosity was determined as the ratio of pore volume to the total volume of the sample, obtained by multiplying 1 cm.sup.2 by the thickness of the sample.

[0121] Experimental Method 2—Permeability

[0122] Unless specified otherwise, the permeability of materials is determined herein according to the following method:

[0123] A square 1 cm.sup.2 sample was cut out and placed on dry filter paper. A drop of an electrolyte (1 M solution lithium perchlorate in sulfolane) was applied by pipette to the top side of the sample and left for 15 min, after which time any electrolyte remaining on the top side of the sample was removed using a cotton bud. The sample was then carefully removed from the filter paper. The presence of through pores, i.e. a permeable material, was confirmed where wetting of the filter paper underneath the sample was observed.

Example 1—Manufacture of a Porous Composite Lithium Electrode (12-14% Porosity)

[0124] A piece of non-woven polypropylene film having opposed faces with dimensions of 4 cm×6 cm and the properties shown in Table 1 was positioned inside a vacuum chamber at a pressure of 0.03 mbar a.

TABLE-US-00001 TABLE 1 Property SpanBel F Thickness (μm) 15 Surface density (g/m.sup.2) 20-25 Porosity v/v (%)   18-20% Permeability Yes

[0125] Metallic lithium was evaporated from an evaporation unit at a temperature of 500° C. The evaporated metallic lithium was deposited on one side of the surface of the piece, which was kept between 20 and 50° C. The deposition rate was 4 mAh per cm.sup.2 per min (Note: 1 g(Li)=3884 mAh). The total amount of metallic lithium deposited was about 4 mAh per cm.sup.2 on each side.

[0126] Following deposition on one side, the piece was turned around and the deposition process was repeated. After deposition, the piece was calendared on a roller press to an overall thickness of 55 μm. The overall electrical capacity of the resulting electrode was determined to be 8 mAh/cm.sup.2. The porosity of the resulting electrode was determined to be 12-14%.

[0127] To facilitate evaporation of metallic lithium where a passivation film was formed, the electric evaporation unit was operated in pulsed mode (on/off), at a frequency of 0.5 Hz.

Example 2—Manufacture of a Porous Composite Lithium Electrode (70% Porosity)

[0128] A piece of non-woven polypropylene film having opposed faces with dimensions of 4 cm×6 cm and the properties shown in Table 2 was positioned inside a vacuum chamber at a pressure of 0.03 mbar a.

TABLE-US-00002 TABLE 2 Property Mechanically modified Span Bel F Thickness (μm) 110 Surface density (g/m.sup.2) 13 Porosity v/v (%) 88 Permeability Yes

[0129] Low vacuum thermal deposition of metallic lithium onto the polypropylene film was carried out according to the general method of Example 1. The amount of lithium deposited was measured as 0.522 mg/cm.sup.2 (per side), which is equivalent to a total amount of 2 mAh per cm.sup.2 (per side). The deposition took about 2 minutes. The porosity of the resulting electrode was determined to be 70%.

[0130] To facilitate evaporation of metallic lithium where a passivation film was formed, the electric evaporation unit was operated in pulsed mode (on/off) at a frequency of 0.5 Hz.

Example 3—Manufacture of a Porous Composite Lithium Electrode (45% Porosity)

[0131] A piece of non-woven polypropylene film having opposed faces with dimensions of 4 cm×6 cm and the properties shown in Table 3 was positioned inside a vacuum chamber at a pressure of 0.03 mbar a.

TABLE-US-00003 TABLE 3 Property Mechanically modified Span Bel F Thickness (μm) 78 Surface density (g/m.sup.2) 13 Porosity v/v (%) 82 Permeability Yes

[0132] Low vacuum thermal deposition of metallic lithium onto the polypropylene film was carried out according to the general method of Example 1. The amount of lithium deposited was measured as 0.722 mg/cm.sup.2 (per side), which is equivalent to a total amount of 3 mAh per cm.sup.2 (per side). The deposition took about 3 minutes. The porosity of the resulting electrode was determined to be 45%.

[0133] To facilitate evaporation of metallic lithium where a passivation film was formed, the electric evaporation unit was operated in pulsed mode (on/off) at a frequency of 0.5 Hz.

Example 4—Comparative—Manufacture of a Metallic Lithium Foil Electrode

[0134] A conventional negative electrode was cut from a smooth lithium foil having a thickness of 60 μm.

Example 5—Comparative—Manufacture of a Reinforced Metallic Lithium Foil Electrode

[0135] A reinforced lithium electrode was prepared according to the method described in the Example of WO 2013/121164.

[0136] A sheet of lithium (Li) foil with a thickness of 60 μm was reinforced using a non-woven polypropylene (PP) sheet having a thickness of 50 μm. A Li/PP/Li composite having an initial thickness of 170 μm was formed and then rolled using steel rolls on a roll press.

[0137] After rolling, the final thickness of the electrode was 60 μm.

[0138] Comparison 1—Physical Properties of Porous and Foil Electrodes

[0139] The porosity, permeability and mechanical strength of the electrodes of Examples 1 to 5 were compared. The results are shown in Table 4:

TABLE-US-00004 TABLE 4 Electrode Example 1 Example 2 Example 3 Example 4 Example 5 Porosity 12-14% 70% 45% 0% 0% v/v (%) Permeability Yes Yes Yes No No Mechanical Good Good Good Poor Good strength

[0140] Comparison 2—Electrochemical Properties of Porous and Foil Electrodes

[0141] The electrochemical properties of the electrodes of Examples 1, 4 and 5 were investigated by measuring their impedance spectra.

[0142] In each case a two-electrode cell was assembled from a pair of each type of electrodes and a separator soaked with electrolyte. Each electrode had opposed faces each having an area of 5 cm.sup.2. The thickness of the separator was 200 to 220 μm. A 1 M solution of lithium perchlorate in sulfolane (15 to 20 μL per cm.sup.2 of one of the opposed faces of one of the electrodes, i.e. 5 cm.sup.2) was used as the electrolyte.

[0143] The electrical impedance of the cell was measured in the frequency range of 25 Hz to 100 kHz using a Solatron impedance spectrometer.

[0144] The results obtained in respect of the cell containing the electrode of Example 1 is shown in FIG. 1(a). In particular, FIG. 1(a) presents the impedance spectra of the cell containing the electrode of Example 1 at 1 h and 20 h after cell assembly.

[0145] For comparison, the impedance spectra of a cell containing electrodes made from the electrode of Example 4 are presented in FIG. 1(b). In particular, FIG. 1(b) presents the impedance spectra of the cell containing the electrode of Example 4 at 1 h and 15 h after cell assembly.

[0146] The impedance spectra for Example 5 were almost identical to that shown in FIG. 1B for Example 4.

[0147] The comparison of FIGS. 1(a) and 1(b) shows that the electrodes produced according to Example 1 have much lower surface resistance, are more stable and less prone to passivation in electrolyte systems.

[0148] Comparison 3—Overvoltage Vs. Number of Cycles of Porous and Foil Electrodes

[0149] The cells of Comparison 2 were exposed to galvanostatic cycling with i=0.5 mA/cm.sup.2 and Qκ=1.0 mAh/cm.sup.2.

[0150] With reference to FIG. 2, the cell produced from the electrode of Example 1 was found to have a better cycle life than the cell produced from the electrode of Example 4.

[0151] Comparison 4—Lithium Surface Morphology According to Varying Porosity

[0152] A scanning electron microscope (SEM) was used to image the lithium surface of electrodes according to Examples 2 and 3. A comparison of the surface morphologies of the electrodes of Examples 2 and 3 is shown in FIG. 3.

[0153] FIG. 3(a) shows the morphology of the lithium surface of a 70% porous electrode according to Example 2. FIG. 3(b) shows the morphology of the lithium surface of a 45% porous electrode according to Example 3.

[0154] Comparison 5—Electrochemical Properties According to Varying Porosity

[0155] The electrochemical properties of lithium composite material with different levels of porosity (Examples 2 and 3) were investigated by galvanostatic cycling and electrical impedance.

[0156] The studies were carried out in a two-electrode electrochemical cell (a so-called half-cell). The working and counter electrodes were manufactured as described in Example 2 and Example 3. All cells incorporated a microporous polypropylene separator, (Celgard 3501™), which was soaked with electrolyte solution made of 1 M lithium perchlorate (LiClO.sub.4) in sulfolane.

[0157] The composition of the working and counter electrodes was as follows: [0158] (1) Li/NPP—lithium deposited on non-woven polypropylene, Q=2 mAh/cm.sup.2, prepared as described in Example 2 (70% porosity). [0159] (2) Li/NPP—lithium deposited on non-woven polypropylene, Q=2 mAh/cm.sup.2, prepared as described in Example 3 (45% porosity).

[0160] The corresponding impedance hodographs are presented in FIGS. 4 (a) and (b), with the following parameters: i=±0.2 mA/cm.sup.2, U=±0.5 V, room temperature.

[0161] The cell assemblies as described above were also cycled in galvanostatic cathodic-anodic mode with the following parameters: i.sub.c=i.sub.a=0.2 mA/cm.sup.2; Q.sub.c=0.3 mAh/cm.sup.2, U=±0.5 V, room temperature. The relationship between overvoltage and cycle number is presented in FIGS. 5 (a) and (b).

[0162] The sample of Example 2 (FIG. 5(a)) demonstrated significantly better cycle life at lower impedance (FIG. 4(a)) than the sample of Example 3 which is characterized by lower porosity.